1. Field of the Invention
The present invention relates to a dental composition for intra-oral impression-material, which is provided in such a powdery or pasty form that, when in the powdery form, it forms an aqueous gel upon being mixed with water, or when in a two-pack paste form, it gels upon mixing. This dental composition is characterized in that it is much superior in storage stability, dimensional stability and accuracy to analogous alginate impression materials and is virtually unreactive with the surfaces of gypsum models.
2. Prior Art
Dental impression materials are generally broken down into non-elastic and elastic types; dental impression materials of this invention belongs to the latter type. Impression materials so far used are based on agar, alginates, polysulfide rubber, polyether, silicone rubber and the like.
The elastic impression material, even when having deformed elastically in removing the impression from the oral cavity, tends to revert to its original form in the absence of stress. Thus, it can be used to take impressions of the teeth, row of teeth, jaw, mucosa and other intra-oral regions, all being morphologically complicated due to the presence of undercuts.
Synthetic rubber-based impression materials formed of such kinds of rubber as polysulfide rubber, polyether rubber and silicone rubber have the properties of being clinically moderate in elasticity, easy to manipulate and low in permanent deformation as well as being reduced or limited in the dimensional change with time and tensile strength of their set products, and so have been used to take precise impressions.
However, the polysulfide rubber impression material is disadvantageous in that it gives off unpleasant smells and is slow-setting, while the polyether rubber impression material has the defects of being not only hard and lower in rubber elasticity but is noticeably affected by moisture as well, though not so much affected as the alginate material. Silicone rubber, by contrast, is now considered the most suitable material for precise impression-taking, because it is tasteless and odorless, sharply set, excellent in elastic properties, extremely limited in dimensional change and improved in dimensional stability. However, the drawback of it is that it is expensive. By and large, such synthetic rubber-based impression materials have a major defect of being costly. Consequently, most dental clinics still rely upon agar and alginate impression materials for such limited purposes as taking one-point impressions for crowns or inlays and general impressions. This is because in spite of their defects of being larger in permanent deformation, richer in moisture so that the resulting impressions can undergo larger dimensional changes with time, and lower in tensile strength so that the resulting impressions can tear up easily, as compared with the synthetic rubber-based impression materials, they are much richer in hydrophilic nature and elasticity so that impression-taking can be facilitated, and, particularly, they are available at low cost. In Japan in particular, agar-alginate combination impressions enjoy increased use.
Among them, the alginate impression material, now used most abundantly, is provided mainly in a powdery form and is designed to gel when mixed with water. In recent years, low-dusting, powdery alginate impression material has been used with a view to improving dental work and working environments. Rendering it completely dust-free, however, is still unfeasible.
For use, given amounts of the powdery alginate impression material and water are kneaded together in a small rubber bowl with the use of a spatula to prepare a paste, which is then inserted into and brought into pressure contact with the oral cavity through an impression tray. After the paste gels into an elastomer, it is removed from within the mouth to take the intra-oral impression. After that, gypsum slurry is poured in the negative-die of the impression to make a working model for preparing prostheses, thereby making a gypsum model. How faithfully the surface details of the gypsum model are reproduced is correlated with the degree that the prepared prosthesis is fitted into the oral cavity. The surface roughness of the gypsum model is then governed by the affinity between or the interfacial relation between the alginate impression material and the gypsum model. For this reason, it is practically of crucial importance to use selectively material that promotes or accelerates the gelation of the alginate impression but does not inhibit, the hardening of gypsum.
As explained above, although the alginate impression material is inexpensive, it is poor in tensile strength and renders the surface roughness of the gypsum model large. For all the numerous studies so far made to eliminate such defects, no satisfactory alginate impression material has yet been developed.
The rubbery impression materials are lower in their dimensional change with time and satisfactory in terms of tensile strength and the roughness of the gypsum model surface, on the other hand, there is a limitation to slashing the cost in consideration of their starting materials themselves being costly. It would thus be impossible to make them comparable to alginate in terms of price.
There is thus a great demand toward an impression material which is as inexpensive as the alginate impression material, and is as low in dimensional change with time and is as satisfactory in terms of tensile strength and roughness of a gypsum model surface as the rubbery impression material.